1. Field of the Invention
The present invention generally relates to processes for the recovery of metals from aqueous solutions derived from various source materials via solvent extraction methods that use metal-specific extraction reagents. In a more specific aspect, the invention relates to improved processes for the recovery of molybdenum and uranium that can be present in low concentration from acidic aqueous solutions.
2. Description of the Related Art
Traditionally, ion exchange is the preferred method used to recover metals from low concentration feeds. However, given the nature of the mining and metal recovery industry, the feed solutions can have a large degree of variability in the concentration of organics, solids and other contaminants. These contaminants are well known to be very problematic and foul the ion exchange resin leading to a reduction in the mass transfer coefficient and less efficient exchange performance. When metals are extracted from low feed concentrations, the resin will be contacted with the aqueous feed for a longer time before reaching full capacity. This leads to a more rapid fouling of the resin and faster decrease of the resin capacity which requires more frequent regeneration cycles. Sometimes fouling of the resins leads to resin breakdown. The regeneration and/or cleaning of the resin is very laborious when the resin is fouled, producing large amounts of waste and even after extensive washings the exchange capacity of the resin can be significantly altered. Additionally, as the resin is more frequently regenerated and/or washed, the physical properties of the resin can be drastically altered.
In contrast, solvent extraction is a well established technology that can be extremely selective toward a specific target metal and is capable of handling significant amounts of solids, cruds, particulates, and organics. One route to recover metals from ores is by contacting the ore with an aqueous solution containing a leaching agent such as an acid which extracts the metal from the ore into solution. The aqueous leach solution, called pregnant leach solution, is then treated via a solvent extraction process wherein the pregnant leach solution is contacted with a non-aqueous (e.g., organic) solution containing a metal-specific extraction reagent. This reagent extracts the metal from the aqueous phase into the non-aqueous phase; the aqueous leaching solution, called raffinate is generally recycled back to the leaching process to dissolve more metal. The aqueous leach solution may contain other metals than the primary metals. For example it has been reported by A. Cruz and A. Reghezza, Hydrocopper 2007, Chapter 07 New projects and expansions, p. 349-355 that molybdenum is found in the different aqueous streams involved in the extraction of copper from copper ores with sulfuric acid. The molybdenum from these streams is valuable and is worth recovering, even though only present in low concentration (e.g., from 1 ppm to 1,000 ppm). While recovery of molybdenum from oxidized molybdenum sources via solvent extraction has been generally investigated, solvent extraction technology has not typically been used in recovering such metals that are present only in low concentrations due to inefficiency.
For example, sources of oxidized molybdenum include oxidized molybdenite and molybdenum resulting from leaching of any molybdenum containing ores. Other sources of oxidized molybdenite include, spent catalyst, recycled alloys containing molybdenum, scrubbing solutions from the roasters, smelting residues, alloys, leaching of molybdenite concentrate, pressure oxidized molybdenite, etc.
Molybdenum solvent extraction from acidic media using amines has been described in U.S. Pat. Nos. 3,455,677; 4,000,244 and 4,444,733 and is used to extract molybdenum from aqueous solutions. However the drawbacks to these methods include silicon transfer in the organic phase which is associated with precipitation during stripping, co-extraction of phosphorus, arsenic, antimony, lead, bismuth and selenium, third phase formation, poor phase disengagement, and also poor solubility of molybdenum-amine complexes. The presence of phosphorus in the aqueous feed leads to the formation of a third phase which has been characterize by Moyer et al., W. J. McDowel, Hydrometallurgy, 1986, 16, p. 177-195 as PMo12O403−.
Another venue explored to recover molybdenum is using bis(2-ethylhexyl)phosphoric acid (DEHPA) or phosphonic acids as described by B. Nyman et al. in Oslo Symposium 1982, Ion Exchange and Solvent Extraction, Ed. Joan Frost Urstad and Gerd Borgen, Society of Chemical Industry , pp. V-15-V-35, 1982. However, these methods suffer from a high level of iron being co-extracted with molybdenum.
Oximes also have been investigated for molybdenum extraction in U.S. Pat. No. 4,026,988, but their application is limited by the reduced stability of the oximes in the basic media which is necessary to fully strip molybdenum from the organic phase.
The use of phosphinic acids to extract molybdenum has been described in JP6192761A; P. Zhang et al., Energy & Fuels 1995, 9, 231-239; M. Oliazadeh et al., Hydrometallurgy 2003-Fifth International Conference in Honor of professor Ian Ritchie-Volume 1: Leaching and Solution Purification, 843-852, 2003; A. Saily et al., Fresenius J. Anal. Chem, 360, 266-270, 1998; P. Behera et al., Journal of Radioanalytical and Nuclear Chemistry, 178(1), 179-192; and Y. Cao et al., Mo Kexue Yu Jishu, 9(4), 6-12, (1989).
Recovery of molybdenum from acidic aqueous solutions using phosphinic acids such as Cyanex®272 (available from Cytec Industries, Woodland Park N.J.) have been described in Japanese patent application JP 6-192761 and P. Zhang et al., Energy & Fuels 1995, 9, 231-239. Zhang et. al reported using a large excess of NH4OH. However, this process has the drawback of significant amounts of phosphinic acids being transferred to the aqueous phase, which results in important losses of extractant as well as significant ammonium and water transfer into the organic phase. Consequently, when the organic phase is recycled back to the extract stage considerable amounts of ammonium salts build up in the acidic aqueous solution. In addition emulsion formation (i.e., phase disengagement) has been reported. Thus, these solvent extraction processes for recovering molybdenum are not practical for use on an industrial scale.
Accordingly, the solvent extraction processes for recovering metals from aqueous solutions from various sources require further improvement. Processes that fine tune metallurgical organic:aqueous (O:A) ratios so as to selectively recover metals present in only trace amounts from existing operations without downstream impact on leaching operations or solvent extraction operations, thereby effectively eliminating separate mining costs for such metals, would be a useful advance in the art and could find rapid acceptance in the metallurgical mining industry. Additionally, solvent extraction processes that use ammoniacal solutions in the stripping step and that do not give rise to phase disengagement issues or emulsion formation would also be a useful improvement.